Thin oxide coatings are widely used in industry, in particular in optics, as protective coatings or for optical functional purposes. For example, they can serve as protection against corrosion and mechanical damage or for coating the surfaces of optical components and instruments, in particular lenses, mirrors, prisms, etc. Thin oxide coatings are furthermore used to produce optical coatings of high, medium and low refractive index for increasing or reducing reflection. The principal areas of application are the production of antireflection coatings on optical substrates such as spectacle lenses, elements for camera lenses, binoculars and optical components for optical measuring instruments and laser technology. Other areas are the production of coating, having a certain refractive index and/or certain optical absorption properties, for example in interference mirrors, beam splitters, heat filters and diathermic mirrors.
Starting materials for the production of oxide coatings of this type are known per se. Usual materials are SiO.sub.2 and a wide range of metal oxides, if desired in combination with one another. The choice is made essentially empirically depending on the target optical properties and the processing properties. The coatings are produced by the vacuum vapor deposition method, which is known per se. For an exemplary illustration, reference is made here to German Patent 12 28 489, which describes the materials which can be used, the processing methodology and the problems which occur in this connection.
For the production of coatings of medium refractive index, i.e., coatings having optical refractive index values of about 1.6 to 1.9(at a wavelength of 500 nm), the range of starting materials which are suitable in principle is limited. Suitable starting materials are essentially the oxides of aluminum, magnesium, yttrium, lanthanum, praseodymium, and also cerium fluoride and lanthanum fluoride, and mixtures thereof.
However, these materials which are suitable per se have a number of disadvantages which become particularly noticeable from a practical point of view during processing.
One aspect here is that these substances have high melting and boiling points, which in addition are relatively close to one another. From a practical point of view, however, it is important that the vapor-deposition materials have melted fully before significant evaporation begins. Only then is a uniform and adequate evaporation rate ensured. This is necessary so that homogeneous coatings of uniform thickness form on the objects to be coated. However, this is not the case under practical use conditions for the oxides of magnesium and yttrium, which melt only incompletely, or not at all, under usual working conditions. They are overall difficult to evaporate, and coatings having variations in thickness are-obtained.
It is therefore desired to lower the melting points of the base materials by means of suitable additives. Additives also serve to vary and set the refractive index in the resultant coating within certain limits in a specific manner.
The choice of suitable additives for these purposes is restricted by the requirement for freedom from absorption. The only metal oxides suitable as appropriate additives are therefore those which do not absorb in the near infrared and in the visible spectral region as far as the near UV wavelength region (approx. up to 200 nm).
The use of oxide mixtures is undesired per se in vacuum vapor deposition technology. The reason is that mixtures generally evaporate incongruently, i e. they change their composition during the evaporation process, and the composition of the deposited coatings and thus their refractive index changes correspondingly. Typical examples thereof are tantalum oxide/aluminum oxide or hafnium oxide/aluminum oxide mixed systems.